U.S. Secretary of State John Kerry, left, and Russia's Foreign Minister Sergei Lavrov shake hands prior to their meeting in Geneva, Switzerland, on Saturday. (AP Photo/Carolyn Kaster, Pool)

GENEVA — Iran struck a historic nuclear deal Sunday with the United States and five other world powers, in the most significant development between Washington and Tehran in more than three decades of estrangement between the two nations.

The agreement commits Iran to curb its nuclear activities in exchange for limited and gradual sanctions relief. It builds on the momentum of the dialogue opened during September’s annual U.N. gathering, which included a 15-minute phone conversation between President Barack Obama and Iran’s new president, Hassan Rouhani.

It marks a milestone between the two countries, which broke diplomatic ties 34 years ago when Iran’s Islamic revolution climaxed in the storming of the U.S. Embassy in Tehran. Since then, relations between the two countries have been frigid to hostile — until the recent outreach between the two presidents.

Obama hailed the deal as putting “substantial limitations” on a nuclear program that the United States and its allies fear could be turned to nuclear weapons use.

“While today’s announcement is just a first step, it achieves a great deal,” Obama said. “For the first time in nearly a decade, we have halted the progress of the Iranian nuclear program, and key parts of the program will be rolled back.”

“Agreement in Geneva,” tweeted U.S. Secretary of State John Kerry, who flew to Geneva on Saturday, joining foreign ministers of the nations negotiating with Iran to push the deal through. “First step makes world safer. More work now.”

Kerry said the first-step deal will make Israel — an arch enemy of Iran — safer. He was trying to pacify Israel’s vehement opposition to the deal.

Israel Prime Minister Benjamin Netanyahu has loudly criticized the agreement, saying the international community is giving up too much to Iran, which it believes will retain the ability to produce a nuclear weapon and threaten Israel.

A White House statement called the nuclear agreement an “initial, six-month step.”

Specifically, the statement said the deal limits Iran’s existing stockpiles of enriched uranium, which can be turned into the fissile core of nuclear arms.

The statement also said the accord curbs the number and capabilities of the centrifuges used to enrich and limits Iran ability to “produce weapons-grade uranium” from a reactor in the advanced stages of construction. The statement also said Iran’s nuclear program will be subject to “increased transparency and intrusive monitoring.”

“Taken together, these first step measures will help prevent Iran from using the cover of negotiations to continue advancing its nuclear program as we seek to negotiate a long-term, comprehensive solution that addresses all of the international community’s concerns,” said the statement.